male -- In organisms with separate sexes, the one which produces sperm.

mammilla -- n. In eggshell, the cone-like structure at the base of the shell unit where the shell unit attaches to the inner organic membrane.

mannoxylic -- Wood in which there is a great deal of parenchyma tissue among the xylem is called mannoxylic. Cycads and pteridosperms have mannoxylic wood. Contrast with pycnoxylic.

mantle -- That portion of the interior of the Earth that lies between the crust and the core.

marine -- Refers to the ocean.

marine terrace -- n. A platform of marine deposits (typically sand, silt, gravel) sloping gently seaward. Such a platform may be exposed along the coast, forming cliffs, due to uplift and/or the lowering of sea level, e.g., marine terraces of coastal Southern California.

marl -- n. A loose, crumbly deposit consisting of clay and calcium carbonate and formed in marine or freshwater conditions.

marsupial -- n. A mammal that gives live birth to young that have gestated for only a short period of time. The young usually crawl into a pouch (the marsupium) or protected area and attach to their motherís teat to finish developing. Examples of marsupials include kangaroos, opossums, and koalas.

mastigoneme -- Small hair-like filaments found on the "hairy" flagellum of the Chromista.

megaspore -- In plants which are heterosporous, the larger kind of spore is called a megaspore; it usually germinates into a female (egg-producing) gametophyte. Contrast with microspore.

Meguma -- n. - A terrane or microcontinent that formed in the Cambrian as part of the continental shelf of Gondwana, rifted apart in the Ordovician, and collided with the Laurentia in the Devonian; it forms the southern part of mainland Nova Scotia (Meguma Zone), separated from the Avalon Zone by a large fault that runs from Cobequid Bay to Chedabucto Bay.

meiosis -- A two-stage type of cell division in sexually reproducing organisms. In meiosis, a diploid cell divides to produce four haploid cells, each with half the original chromosome content. For this reason, meiosis is often called a "reduction division". In organisms with a diploid life cycles, the products of meiosis are usually called gametes. In organisms with an alternation of generations, the products of meiosis are caled spores.

melange -- A body of rocks consisting of large blocks (mappable size) of different rocks jumbled together with little continuity of contacts.

membrane -- In biology, a boundary layer inside or around a living cell or tissue.

meristem -- Group of undifferentiated cells from which new tissues are produced. Most plants have apical meristems which give rise to the primary tissues of plants, and some have secondary meristems which add wood or bark.

merophytes -- Group of cells which have all been produced from the same initial cell. Leaves and stems in particular are often built from specific patterns of merophytes.

mesoderm -- In animals with three tissue layers (i.e. all except sponges and cnidarians), the middle layer of tissue, between the ectoderm and the endoderm. In vertebrates, for instance, the mesoderm forms the skeleton, muscles, heart, spleen, and many other internal organs.

mesogloea -- Jellylike material between the outer ectoderm and the inner endoderm of cnidarians. May be very thin or may form a thick layer (as in many jellyfish).

mesokaryotic -- Nuclear condition unique to the dinoflagellates in which the chromosomes remain permanently condensed.

metabolism -- n. The chemical processes within an organic body that supply the energy necessary for life. The rate of metabolic processes is sometimes used as a way to differentiate organisms. For example, mammals generally have a higher metabolism than reptiles and can thus sustain higher levels of activity for longer periods of time.

metamorphic rock -- Any rock derived from other rocks by chemical, mineralogical and structural changes resulting from pressure, temperature or shearing stress.

metamorphism -- n. The process of altering the chemical or mineralogical composition of a rock through different amounts of heat and pressure below the surface of the Earth; metamorphose- v; metamorphic - adj.

metamorphosis -- n. A process of developmental change whereby a larva reaches adulthood only after a drastic change in morphology; occurs in most amphibians and insects, for some insects, this change may include another stage (pupa) before the adult stage; metamorphose- v.

microfossil -- n. A very small fossil, best studied with the aid of a microscope, e.g. foraminifera, radiolarians, and small vertebrate fossils such as teeth. macrofossilĖ ant.

microphyll -- A kind of leaf, specifically one which has a single, unbranched vein in it. Microphylls are only found in the lycophytes.

microscopic -- Objects or organisms that are too small to be seen with the naked eye.

microspore -- In plants which are heterosporous, the smaller kind of spore is called a microspore; it usually germinates into a male (sperm-producing) gametophyte. Contrast with megaspore.

microstructure -- n. In eggshell, the shape, size, orientation, and distribution of components of the shell.

microtubules -- Type of filament in eukaryotic cells composed of units of the protein tubulin. Among other functions, it is the primary structural component of the eukaryotic flagellum.

microvilli -- Thin fingerlike protrusions from the surface of a cell, often used to increase absorptive capacity or to trap food particles. The "collar" of choanoflagellates is actually composed of closely spaced microvilli.

mineralization -- The process of replacing any organismís original material with a mineral.

mitochondrion -- Complex organelle found in most eukaryotes; believed to be descended from free-living bacteria that established a symbiotic relationship with a primitive eukaryote. Mitochondria are the site of most of the energy production in most eukaryotes; they require oxygen to function. See: double membrane.

mitosis -- The process of nuclear division in eukaryotes. It is one step in cytokinesis, or cellular division. MORE ?.

molds -- Fossils formed when the sediment surrounding a buried organism hardens. When the organism decays, its impression is left in the rock and can be seen if the rock is broken open.

monophyletic -- Term applied to a group of organisms which includes the most recent common ancestor of all of its members and all of the descendants of that most recent common ancestor. A monophyletic group is called a clade. More?

monotreme -- n. A mammal that lays eggs rather than giving live birth. Though laying eggs is a primitive reptilian trait, monotremes share many morphological, physiological, and reproductive characteristics with other mammals, making them true mammals. Extant monotremes include the duck-billed platypus and echidna.

monsoon -- n. - A seasonal weather pattern where winds, and often rain, come consistently from one direction for many months. It is caused by the temperature differences between land and ocean, and in general, a larger landmass makes a greater difference, which makes a more extreme monsoon.

monsoonal -- adj. Describes a climate pattern with a wind system that changes direction with the seasons; this pattern is dominant over the Arabian Sea and Southeast Asia.

moraine -- n. A mound or ridge of sediment deposited by a glacier; lateral moraine- n. deposited to the side of a glacier; terminal moraine- n. deposited to the front of a glacier; ground moraine- n. deposited on the land surface.

morphology -- n. The form and structure of anything, usually applied to the shapes, parts, and arrangement of features in living and fossil organisms.

morphotype -- n. An individual or set of organisms within a population distinguished by having a distinct physical structure.

motile -- Able to move oneself about, capable of self-locomotion.

mouth -- Front opening of the digestive tract, into which food is taken for digestion. In flatworms, the mouth is the only opening into the digestive cavity, and is located on the "belly" of the worm.

MTOC -- (microtubule organizing center) MTOCs are bundles of protein tubes which may be found at the base of a eukaryotic flagellum. In animals, they also function in creating the arrays of microtubules that pull the chromosomes apart during mitosis.